In order to use wavelength division multiplexing techniques in optical transmission systems, one requires efficient, narrow-band optical wavelength filters. In a paper entitled "Grating-assisted InGaAsP/InP vertical codirectional coupler filter" by R. C. Alferness et al., published in the 6 Nov. 1989 issue of Applied Physics Letters 55(19), pp. 2011-2013 there is described a four-part channel-dropping filter employing grated-assisted coupling between asynchronous optical waveguides. However, such devices are polarization dependent and will respond differently for the TE and TM modes. As a consequence, the amplitude of the dropped channel will fluctuate in such devices as the polarization of the input signal varies. Accordingly, it is the object of the present invention to provide optical filters and couplers that operate independently of the state of polarization of the incident light.